In U.S. Pat. No. 4,736,101 there is disclosed a method of performing MS/MS in a quadrupole ion trap. In this method a wide mass range of ions are created and stored in the ion trap during an ionization step of the analysis in a manner similar to that disclosed in U.S. Pat. No. 4,540,884. According to the equations which govern operation of quadrupole ion traps, ions of differing masses will have distinct and unique natural frequencies of oscillation in the ion trap. By application of a small supplemental ac voltage at this frequency applied by a synthesizer circuit, or the like, to the end caps of the electrodes of the ion trap, selected ions are caused to resonate and either enter into a collisionally-induced dissociation or are ejected from the ion trap. All other ions which have different masses remain unaffected by the supplemental ac field. Those ions which undergo collisionally-induced dissociation form daughter ions which are then trapped and can be scanned out of the device by ramping or increasing the r.f. voltage applied to the ring electrode as disclosed in U.S. Pat. No. 4,540,884.
Only a single mass is excited at any given time. For tandem mass spectrometry (MS/MS) this provides only daughter ion analysis of a single parent mass. The parent ion is selected by selecting the frequency of the applied supplemental voltage and the daughter ions are ejected by scanning. It is possible to obtain a reconstructed parent ion spectrum from a large number of successive daughter ion scans, each with its own ionization event. The time involved in acquiring the entire data set would prevent the use of this approach for obtaining parent ion information during routine mixture analysis.
In tandem mass spectrometry, three useful scan modes are the parent scan, neutral loss scan and selected or multiple reaction monitoring. The first two modes are very useful for screening analytical samples for the presence of specific classes of compounds, while the latter scan mode is useful for screening with high sensitivity the presence of specific compounds. These modes have not been implemented on quadrupole ion trap mass spectrometers.
In co-pending application Ser. No. 645,622 entitled, "QUADRUPOLE ION TRAP MASS SPECTROMETER HAVING TWO AXIAL MODULATION EXCITATION INPUT FREQUENCIES AND METHOD OF SCANNING," filed simultaneously herewith (FHTAH File No. A-54111/AJT), there is described an implementation of parent and neutral loss scans using an ion trap mass spectrometer. The apparatus and method involves the simultaneous application of two resonant excitation waveforms: one for CID of a series of parent ions, and the other for ejection of the resulting daughter ions of interest.
In certain modes of operation, the CID of the parent ion forms daughter ions which are not trapped and some ejection of parent ions which do not undergo CID occurs. These events, in sum, contribute to the ion current being detected at the frequency of the parent ion of interest.